Right click and edit the data set. Can't remember what the options are exactly (View Source Data or something?), but it should be somewhat obvious. You can define what you want the X and Y values to be there.

Thankyou for your tip, however:
I could just be completely stupid but I can't seem to find anything like what you are talking about. I did find one window (under the Chart menu -> Source Data) which may be what you mean but that didn't have anything about defining the X and Y values (at least I couldn't see anything).

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